This invention relates to a method of improving the uniformity and quality of the base insulating coating on silicon-iron steel. More particularly, this invention relates to an annealing separator coating composition and a method of producing an annelaing separator coating on silicon-iron steel strip.
Such silicon steel or silicon-iron steel is useful for its electrical and magnetic properties and may include both oriented and nonoriented steels. In the production of such steels, an annealing separator coating may be used to improve the magnetic properties and prevent sticking of coil laps during heat treatment. Annealing separator coatings are particularly useful with grain oriented silicon steels.
Grain oriented silicon steel is used in various electrical applications, such as transformers and the like. The desired cube-on-edge grain orientation is produced during a final high temperature annealing operation. Prior to the annealing operation and after hot rolling, the steel is pickled, cold rolled to final gauge by a series of cold rolling operations with intermediate anneals, decarburized and then final high temperature annealed to achieve the desired secondary recrystallization and cube-on-edge texture. The secondary recrystallization is achieved by inhibiting primary grain growth during the stages of the annealing operation wherein this occurs. This is conventionally achieved by providing primary grain growth inhibitors, such as boron, manganese sulfides and aluminum nitrides.
Prior to final texture annealing, the steel is conventionally coated with an annealing separator coating, such as magnesium oxide. This coating may be applied in the form of a water slurry, or electrolytically, to the surfaces of the strip. The strip is then typically wrapped in coil form for annealing. Final texture annealing is performed at temperatures on the order of 2200.degree. F. (1404.degree. C.). The annealing separator coating prevents the convolutions of the coil from bonding together during the high temperature annealing treatment, and in addition reacts with the silica present on the surface of the sheet to form a strong forsterite insulating film. Also, the coating improves magnetic properties of the silicon steel by removing sulfur after secondary recrystallization has taken place. The sulfur acts as an inhibitor, like boron, to primary grain growth during texture annealing.
Moisture present in the magnesium oxide coating as magnesium hydroxide is liberated to cause transient oxidation of the steel surface as some of the iron is reacted therewith to form iron oxides. This results in irregular coating with the strip having uncoated areas, as well as deposits of reduced iron oxides on the surface of the strip. This poor surface quality impairs the performance of the steel in final electrical product applications.
Attempts have been made by others to improve the annealing separator coating. U.S. Pat. No. 3,544,396, issued Dec. 1, 1970, discloses adding 1 to 20% of chromic oxide (Cr.sub.2 O.sub.3) by weight to a glass-forming magnesia annealing separator. The chromic oxide is an active additive which is disclosed to react with the silicon in the steel to form silica which reacts with the magnesia to form a more continuous silicate glass on the steel surface. The chromium metal is to diffuse into the silicon steel. Other additives, such as calcium oxide (CaO), are also disclosed to be reactive for the silicate glass formation.
U.S. Pat. No. 3,615,918, issued Oct. 26, 1971, relates to a method of producing an insulating glass coating using about 1-25% by weight of decomposable phosphate compounds in the annealing separator (magnesia) coating.
U.S. Pat. No. 3,956,029, issued May 11, 1976, discloses a magnesia annealing separator having an adjusted particle size distribution of the magnesia particles so as to provide the silicate glass formation and to maintain the friction between the steel sheets such as to prevent deformation of the steel during annealing. Magnesium compounds, as magnesium hydroxide, are disclosed as burned, to produce particles having a bulk density of between 0.18 and 0.30 g/cm.sup.3 and a particle distribution of 40 to 70% not larger than 3 .mu.m and not more than 15% of coarse particles larger than 15 .mu.m.
It is, accordingly, a primary object of the present invention to provide a method for coating grain oriented silicon steel prior to final texture annealing wherein an improved coating is obtained and the adverse effects of liberated water are avoided.
Further, an object is to substantially eliminate the iron oxidation on the strip resulting from moisture between the coil laps.
It is also an object to improve the base coating development to provide better uniformity and quality of the coating.
This and other objects of the invention, as well as a more complete understanding thereof, may be obtained from the following description and specific examples.